The last mile in a communications network can present many problems that are quite unique, and are very much unlike those encountered by the more extensive cross-country links of the network. Specifically, significant portions of this last mile are typically in an urban environment wherein there are requirements that land-line links be regulated and routed in accordance with specified codes. An alternative to the complications confronted by land-line links within the last mile is to use line-of-sight wireless systems which can transmit data directly from one station to another through free space.
Presently it is known that there are two types of wireless systems which are particularly useful for establishing line-of-sight communications links through free space. These are: 1) an optical wireless system, and 2) a microwave system. Both, however, have their respective advantages and disadvantages. Specifically, an optical wireless communications system has a relatively high data transmission rate. In an order of magnitude, the data transmission rate on an optical wireless system is around ten times higher than the data transmission rate on a microwave system. On the other hand, a microwave system is typically more robust than an optical wireless system in that its effective operation is far less susceptible to interference from atmospheric conditions such as fog and precipitation. Further, in addition to the more obvious atmospheric conditions that can hamper the operation of an optical wireless communications system, the sun can also have an adverse effect on such an operation by increasing the noise level of the communications channel when it is receiving direct sunlight. Moreover, the effect of the sun may be asymmetric, and hamper the operation of one station in the line-of-sight communications link while other stations may remain unaffected.
A meaningful measure of the effectiveness of the data transmission rate of a laser communications system is its bit error rate (BER). As is well known, bits (abbrev. for BInary digiT) are single occurrences in a code, or language, which employs only two kinds of characters (e.g. 0 and 1). While not all bits are absolutely necessary for an effective communication, there is a limit to how far the data can be degraded. Stated differently, there is a BER above which there is no longer an effective communication. It happens that, for a laser communications system, the BER can be determined in several ways known in the pertinent art. One way is by measuring the received signal strength intensity (RSSI) of the laser beam, and then calculating the BER from the RSSI.
Absent a prolonged blockage of its beam path, such as might be caused by a window washer or some other barrier, it is possible to continue the operation of a line-of-sight communications system under most atmospheric conditions. To do this effectively, however, it is necessary for there to be a reliable device that will appropriately switch between an optical wireless system (higher data transmission rate) and a microwave system (more robust). Additionally, the switching device should be capable of ignoring brief interruptions in the communications beam, such as might be caused by the flight of a bird. At all times, however, it needs to favor communications on the optical wireless system because it has the higher data transmission rate. Nevertheless, in any event, the higher data transmission rate optical wireless system should be used only when it is capable of stabilized operation.
In light of the above it is an object of the present invention to provide a device and method for operationally switching between line-of-sight communications systems which will optimize use of the system having the higher data transmission rate. Another object of the present invention is to provide a device and method for operationally switching between line-of-sight communications systems which maintains an effectively uninterrupted communications link despite brief interruptions and degradations caused by atmospheric conditions. Still another object of the present invention is to provide a device and method for operationally switching between line-of-sight communications systems which will account for low signal-to-noise ratios such as might be caused by sunlight saturation. Yet another object of the present invention is to provide a device and method for operationally switching between line-of-sight communications systems which is easy to use, relatively simple to manufacture, and comparatively cost effective.